Prior switched reluctance motor generally comprise a housing which contains a number of stator poles wound with electromagnetic coils and a rotor with complementary components. The stator poles are often integrally formed with or very much a part of the motor's support structure, the support structure being required to complete the magnetic flux paths between stator poles and rotor components that are necessary for production of motor torque. This precludes any extensive use of light-weight, non-magnetic materials. A significant mount of heat is generated within the motor housing, and steps must be taken to generate and direct air flows through the interior of the motor housing to dissipate heat, particularly in the windings and stator poles. These factors complicate manufacture of a motor and add considerably to cost, size and weight. Since requirements for back-iron and cooling increase significantly with the number of motor poles, it is comparatively rare for a commercial switched reluctance motor to be manufactured with six or more poles.
The stator poles and complementary rotor elements of a switched reluctance motor will normally have a different pitch. The general object is to ensure that one stator pole always overlays one rotor element so that a motor torque initiating rotor rotation can at any time be generated. The natural consequence of such an arrangement is relatively low torque during start-up under loads. To compensate for inherent low starting torques, manufacturers will normally oversize a motor relative to steady-state torque requirements. This further contributes to excessive weight and cost, and also to poor motor efficiency during steady-state operation. Motor efficiency is further reduced by requiring long magnetic flux paths through motor support structure.
The basic motor geometry necessary to ensure self-starting creates additional problems. In steady-state operation only one pole in most commercially available switched reluctance motor can be energized to produce a motor torque. This results in considerable torque ripple, especially given the limited number of poles commonly associated with such a design. This arrangement also produces localized and unbalanced forces in the motor housing and support structure, which must be accommodated with a more robust support structure, further contributing to the weight of the motor.
Lastly, many prior switched reluctance motors involve commutators or slip rings. Such components further complicate manufacture and add to the cost of a motor. Also, these components are subject to excessive wear and tend to require periodic maintenance.